A few months ago, when I sent an e-mail to NASA astronaut Sunita Williams, I got this response:

I am out of the office aboard the International Space Station for the next few months. I will not have access to this account until after I land. Therefore, I will not be responding to any email sent to this account.

Thanks, S

I know Sunny, as we call her, from Needham High School in the suburbs of Boston. We were neighbors; our parents played bridge, and our mothers exchanged recipes.

While Sunny was not actively e-mailing on this account, what I learned from The Week magazine was that she was running the Malabo triathlon 240 miles above Southern California.

It's not easy to swim, bike and run in space. Well, it can be if you cheat and take advantage of the antigravity. But Sunny used equipment to take part and simulate the triathlon. The magazine reported: "For the half-mile 'swimming' portion, Williams strapped into something called the Advanced Resistive Exercise Device (ARED), which uses weights to imitate water resistance while swimming through anti-gravity. For the 18-mile biking portion, she used a stationary bike, and for the 4-mile run, she used a specially outfitted treadmill that strapped her in to keep her from floating off."

After 1 hour, 48 minutes and 33 seconds, a pretty good time, she just had to say what we would say: "I am happy to be done." This wasn't the first big space exercise accomplishment. She also did the Boston Marathon in 2007 in a respectable time of 4:23:10.

What is encouraging is that half of Americans exercise regularly three times or more per week. Yet when we compare by age, 7 percent fewer middle-age Americans (ages 45 to 64) exercise compared to younger Americans (ages 18 to 29.)

A recent study in the Archives of Internal Medicine found that being fit or becoming fit in the middle of life adds years to your life as well as delays chronic diseases such as diabetes, heart disease and cancer.

The study analyzed medical records of nearly 19,000 healthy people in Dallas, Texas, in the 1970s with an average age of 49. These patients received a treadmill test to rate their level of fitness. Medical records were analyzed again 30 years later.

The study found that both fit and unfit individuals, while in middle age, developed the same illnesses over time; yet the fit people had a delay of the chronic illness by 10 to 20 years. In essence, exercising compresses the years of chronic illness, and significantly improves quality of life in the early elderly retirement years.

Even a slight increase in exercise, 20 to 30 minutes of walking on most days, which shifts people from the least fit to more fit, was sufficient to improve long-term health outcomes.

For me and millions of other middle-age folks on Earth, Sunny, who is 47 years old, is an inspiration. While I find it difficult to spare 30 minutes three times a week to swim, Sunny has found time to prepare for and run the triathlon while she in space. Maybe it's pretty boring up there, but more likely, it is her commitment.

So as you sit at the dinner table for a holiday dinner, or on the couch to watch the football games or in the car to go to the grocery store, think about Sunita Williams in space. We can be doing something else: exercising our weight down, living longer and delaying chronic illness.

Manoj Jain is an infectious disease doctor in Memphis who also writes for The Washington Post. Archived articles are available at MJainMD.com.